It is well known that communication systems are a critical part of both commercial and military applications. In fact, as semiconductor technologies advance, the demand for digital communication systems continues to grow. Digital communication systems can be conceptually divided into systems that transmit and systems that receive. A particularly important digital receiver system is the cellular basestation receiver.
Many communication standards (such as EDGE/GSM used in the United States and Europe) specify the generation of multi-carrier signals at transmission frequencies of 850 MHz, 900 MHz, 1800 MHz and 1900 MHz. These transmission frequency multi-carrier signals have a bandwidth of approximately 15 MHz or more and can include any number of single carrier signals. One conventional approach to receiving these signals involves dedicating a receiver system to each single carrier signal. Under this approach, each receiver system can be designed to better handle the anticipated power level of the corresponding single carrier signal, resulting in less stringent dynamic range requirements being placed on the corresponding analog to digital (A/D) converter. The overall systems costs, however, are extremely high because a receiver system is required for each carrier (or channel).
FIG. 1 illustrates another conventional approach to receiving multi-carrier signals. Under this approach, a multi-channel receiver system 10 attempts to process a multi-carrier signal 12. Due to the high transmission frequencies, however, a “down conversion” process is typically required before conversion of the signal into a digital format. Thus, the receiver system 10 mixes the transmission frequency multi-carrier signal 12 with a first local oscillator (LO) signal 14 to generate a preliminary intermediate frequency (IF) multi-carrier signal. A bandpass filter 16 filters the predetermined transmission bandwidth (typically 15 MHz) from the preliminary IF multi-carrier signal. A second down converter mixes a second LO signal 18 with the filtered preliminary IF multi-carrier signal to generate the final IF multi-carrier signal that is used by the analog to digital (A/D) converter 20 to generate a digital signal.
It is important to note that within the typical multi-carrier cellular spectrum there is a large difference in signal amplitudes from carrier to carrier. Thus, given modern day cellular communication protocols, the A/D converter would be required to have an approximately 90 dB dynamic range in order to process the IF multi-carrier signal. Commercially available A/D converters, on the other hand, have a dynamic range that is typically much lower than would be required under the approach shown in FIG. 1 (approximately 60 dB dynamic range). In fact, this dynamic range limitation is a primary reason why the current state of the art in cellular basestations is to have many separate single channel receivers.
It is also important to note that the second down conversion is required because the conventional A/D converter 20 is unable to process data at a rate as high as the preliminary intermediate frequency of 200 MHz. The additional RF circuitry required for the down conversion adds to the overall cost of the system. It is therefore desirable to provide a receiver system that does not require the RF circuitry associated with multiple frequency down conversions. It is also desirable to provide a receiver system that does not require a dedicated A/D converter for each signal carrier signal and does not require an A/D converter with a large dynamic range.